Locking bone screw and spinal plate system

ABSTRACT

A bone plate system including a bone plate and a bone screw with an integrated locking mechanism is disclosed. The bone screw includes an elongate member having a threaded shank and a bone screw head that is radially deformable. The integrated locking mechanism sits within the bone screw head and can rotate between a locked condition and an unlocked condition. When the bone screw head is seated within an aperture in the bone plate, rotating the integrated locking mechanism into the locked condition helps to prevent bone screw backout.

FIELD

The present invention relates to fixation devices used in orthopaedicand spinal surgery and particularly to bone fixation plate systems thatinclude locking bone screws to prevent bone screw backout.

BACKGROUND

For a number of known reasons, bone fixation devices are useful forpromoting proper healing of injured or damaged vertebral bone segmentscaused by trauma, tumor growth, or degenerative disc disease. Thefixation devices immobilize the injured bone segments to ensure theproper growth of new osseous tissue between the damaged segments. Thesetypes of bone fixation devices often include internal bracing andinstrumentation to stabilize the spinal column to facilitate theefficient healing of the damaged area without deformity or instability,while minimizing any immobilization and post-operative care of thepatient.

One such device is an osteosynthesis plate, more commonly referred to asa bone fixation plate, that can be used to immobilize adjacent skeletalparts such as bones. Typically, the fixation plate is a rigid metal orpolymeric plate positioned to span bones or bone segments that requireimmobilization with respect to one another. The plate is fastened to therespective bones, usually with bone screws, so that the plate remains incontact with the bones and fixes them in a desired position. Bone platescan be useful in providing the mechanical support necessary to keepvertebral bodies in proper position and bridge a weakened or diseasedarea such as when a disc, vertebral body or fragment has been removed.

It is known that in bone plate systems in general, and in those systemsused for stabilization of the spinal column in particular, a looseningof the bone screws which secure the bone plate to the bone segment canoccur. When the bone screws become loose, they may move in an axialdirection (i.e., screw back out may occur).

Conventional bone plate systems offer several options for securing thebone screws to the bone plate and preventing screw backout. For example,some systems rely on split rings positioned between the bone screw andthe bone plate; other system use bone screw covers that are mated to thebone plate in a position above an implanted bone screw; and still othersystems use a locking screw that is driven into the top of an implantedbone screw. Despite the existence of these bone plate systems, thereremains a need for an effective bone screw locking mechanism that can beinstalled and actuated with ease and efficiency.

SUMMARY

Disclosed herein is a bone screw with an integrated locking mechanismthat helps to prevent bone screw backout after implantation. The bonescrew and locking mechanism are effective and easy to use. In addition,the bone screw can be implanted and the locking mechanism engaged with aminimal number of steps. For example, the bone screw can be implantedand the locking mechanism engaged with the same tool.

In one embodiment, the bone screw comprises an elongate member having athreaded shank and a head at a proximal end thereof. The head, which isradially deformable, is defined by an outer wall that defines an innerhollow region. An inner surface of the outer wall has a circumferentialgroove that seats a screw locking mechanism. The screw locking mechanismcan be rotated between a locked condition and an unlocked condition.

In one aspect, at least one axially oriented slot is formed in the outersurface of the head, extending distally from the proximal end of thescrew. In an unlocked condition, locking features of the lockingmechanism are aligned with the slot(s) to permit radial deformation ofthe head. In the locked position, the locking features abut the innersurface of the outer wall of the head to prevent radial deformation ofthe head.

At least a portion of the bone screw head has a spherically shaped outersurface that is interrupted by the axially oriented slots formed in theouter wall. The slots, as noted above, allow the bone screw head todeform, for example to reduce its diameter. The locking mechanism canhave a substantially circularly shaped first portion that is adapted tobe rotatably disposed within the seating groove, and a second portion,proximal to the first portion, that includes at least one lockingfeature adapted to engage the inner surface of the outer wall in alocked condition. In addition, the bone screw head can have a drivefeature formed at a distal portion of the hollow region and the lockingmechanism can have a drive feature formed in a central portion thereof.The drive features are adapted to mate with complementary drive elementson a driver tool.

In another aspect, a bone plate system includes at least one bone screwof the type noted above with an integrated locking mechanism, and a boneplate. The bone plate has a first surface, a second, bone-contactingsurface opposed to the first surface, and at least one apertureextending through the first and second surfaces. The aperture has apredefined shape and size, and it is configured to seat a bone screwsuch that the head of the bone screw undergoes radial deformation, atleast upon initial passage into the aperture.

At least a portion of the locking mechanism is rotatably disposed in theseating groove within the bone screw head such that the lockingmechanism can be rotated relative to the bone screw between a lockedcondition and an unlocked conditions in one embodiment, the unlockedcondition allows radial deformation of the head, and the lockedcondition prevents radial deformation of the head by way of a lockingfeature that abuts a portion of the inner surface of the outer wall.

The bone screw and locking mechanism each can include drive features.For example, the head of the bone screw can have a first drive featureformed at a distal portion of the hollow region and the lockingmechanism can have a second drive feature. In one aspect, the first andsecond drive features are positioned coaxially with the first drivefeature positioned distally to the second drive feature.

In one embodiment, the bone plate system includes a driver tool adaptedto mate with the first and second drive features to implant the bonescrew and to actuate the locking mechanism. The driver tool can includea proximal handle portion and a distal mating area that includes a firstdriver element adapted to mate with the first drive feature and a seconddriver element adapted to mate with the second drive feature. Theproximal handle portion can include first and second handle portionscapable of selective independent movement such that one handle can berotated relative to the other. In one embodiment, the driver too candrive the bone screw into bone and subsequently the second drive featurecan be independently rotated to actuate the locking mechanism withoutremoving the tool.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a perspective view of an exemplary embodiment of the boneplate system including a bone plate with bone screws having anintegrated locking mechanism;

FIG. 2A is a perspective view of an embodiment of a bone plate usefulwith the present system;

FIG. 2B is a sectional side view of the bone plate shown in FIG. 2Aalong lines 2B-2B;

FIG. 2C is a partial, sectional side view of an aperture of the boneplate of FIG. 2A along lines 2B-2B;

FIG. 2D is a sectional side view of a portion of a bone plate having abone screw disposed therein;

FIG. 3 is a perspective view of an embodiment of a bone screw usefulwith the system of FIG. 1;

FIG. 4A is a perspective view of an embodiment of a locking mechanismthat can be integrated within the bone screw described herein;

FIG. 4B is a perspective view of the locking mechanism of FIG. 4A in alocked condition;

FIG. 4C is a perspective view of an embodiment of a bone screw adaptedto receive the locking mechanism of FIGS. 4A and 4B;

FIG. 4D is a perspective view of the bone screw of FIG. 4C with thelocking mechanism of FIG. 4A integrated therein;

FIG. 4E is a perspective view of the bone screw and locking mechanism ofFIG. 4D with the locking mechanism in a locked condition;

FIG. 5A is a perspective view of an embodiment of a locking mechanismthat can be integrated within the bone screw described herein;

FIG. 5B is a top view of the locking mechanism of FIG. 5A;

FIG. 6A is a top view of an embodiment a bone screw with a lockingmechanism in a locked condition;

FIG. 6B is a top view of the bone screw of FIG. 6A with a lockingmechanism in an unlocked condition;

FIG. 7A is an embodiment of an installation/locking instrument usefulwith the bone plate system;

FIG. 7B is a perspective view of the distal end of the driver tool shownin FIG. 7A; and

FIG. 7C is a perspective view of the distal end of the driver tool shownin FIG. 7A in an alternative configuration.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those skilled in the art will understand that the devices andmethods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope of the present invention is defined solely by the claims. Thefeatures illustrated or described in connection with one exemplaryembodiment may be combined with the features of other embodiments. Suchmodifications and variations are intended to be included within thescope of the present invention.

The following exemplary embodiments are described herein with referenceto bone screws with bone plates that span and immobilize adjacentvertebral bodies in spinal fixation techniques. However, it isunderstood that the bone screws and bone plate systems described hereinmay be applicable to the fixation of any type of adjacent bones or bonesegments.

FIG. 1 illustrates one embodiment of the bone plate system 10 includingbone screws 1 and bone plate 14 having apertures 16. Bone screws 12 areimplanted through apertures 16 to fix bone plate 14 to bone (e.g.,vertebral bodies). Each bone screw 12 includes a locking mechanism 18that is movable between locked and unlocked positions. In the unlockedcondition bone screw 12 is able to pass into an aperture within theplate and in the locked position the bone screw head is prevented frombacking out of the aperture. The locking mechanism can be integrated(e.g., pre-assembled) within the screw head, or it can be assembledwithin the screw head on demand.

Bone screw 12 with locking mechanism 18 can be locked within a varietyof bone plates that include apertures 16 having shapes and dimensionssuitable to receive bone screw 12 and enable the bone screw to be lockedtherein. Exemplary plates include bone plates having an aperture 16 withan upper diameter 20 sized such that bone screw 12 with lockingmechanism 18 in the locked condition is not able enter or exit thoughupper diameter 20.

FIGS. 2A and 2B illustrate one such exemplary bone plate 14. As shown inthe sectional side view of bone plate 14 provided in FIGS. 2B and 2C,aperture 16 has a variable diameter. The upper portion 22 of aperture 16can have an upper diameter 20 while the lower portion 24 includes alower diameter 26. Positioned between the upper and lower portions 22,24 is a central portion 28 with a central diameter 30 that is largerthan the upper and lower diameters 20, 26. As shown in FIG. 2D, the bonescrew head can be locked within the larger diameter central portion 28of the bone screw aperture 16.

FIG. 3 shows an exemplary bone screw 12 for positioning within aperture16 that includes an elongate body 32 having a distal shank 34 andproximal head 36. The distal shank 34 can include threads 38 for fixingbone screw 12 to bone. A variety of bone screw threads adapted forfixing the bone screw and/or taping bone can be positioned on all or aportion of shank 34. In addition, one skilled hi the art will appreciatethat a variety of thread patterns and sizes can be used.

Bone screw head 36 can have a variety of shapes including partiallyspherical, tapered, and irregularly shaped. In one embodiment, head 36has a generally spherical shape corresponding to the shape of thecentral portion 30 of aperture 16. In another embodiment, the shape ofhead 36 includes a tapered top portion and a tapered bottom portion.

The head 36 of the bone screw 12 includes a wall 42, with an innersurface 44 and outer surface 46, that defines a hollow interior region48. The bone screw head can be constructed in a variety of ways. In oneembodiment, however, it should be able to deform radially.

In one aspect, the screw head is not able to fit within an aperture ofthe bone plate in its normal configuration. However, at least a portionof wall 42 is deformable, and the deformation enables the head of thescrew to pass through and be seated in an aperture. For example, thewall 42 can be deflected inwardly when a compressive force (i.e., causedby passing the screw head through an aperture) is applied to the outersurface 46. Wall 42 may also be somewhat resilient such that after acompressive force is removed from the wall 42, the outer wall is able toreturn to its original shape and dimensions.

Generally, the bone screw head should be capable of deforming by amagnitude sufficient to allow it to fit within an aperture of a plate,for example, the outer diameter of the head can be reduced byapproximately 0.001 to approximately 0.5 mm. The amount of deformationcan dependent on the material(s) used to construct the bone screw andthe type of structure into which the bone screw will be implanted. Inone embodiment, the bone screw is a titanium bone screw sized forinsertion into a cervical vertebra and the outer diameter of the headcan be reduced by approximately 0.001 to approximately 0.25 mm.

To enable deformation, wall 42 can, in one embodiment, include axiallyoriented slots 50 to facilitate deformation of the head. As shown inFIG. 3, slots 50 can extend through wall 42 from the outer surface 46 toinner surface 44. In addition, the slots can extend distally from theproximal-most surface of the head through most of the height of outerwall 42 to define individually deflectable tabs 52. The space providedby slots 50 allows tabs 52 to deform (i.e., flex inwardly or outwardly)and to thereby alter the outer diameter of bone screw head 36 defined bywall 42. One skilled in the art will appreciate the width of the slotsand the number of slots can vary depending on the desired amount ofdeflection in the outer wall. In one embodiment, however, four slots 50are formed, thereby creating four tabs 52.

The hollow interior region 48 of bone screw head 36 can includeadditional features, such as a drive feature that is complementary witha driver element on a driver tool. For example, in the embodimentillustrated in FIG. 3, a drive feature 54 is positioned in the bottomsurface of hollow interior region 48 in the form of a rectangularlyshaped female drive feature. While the drive feature 54 illustrated inFIG. 3 is square, one skilled in the art will appreciate that it canhave a variety of shapes capable of receiving a complementary driverelement. For example, drive feature 54 can have other rectangular shapesor it can be triangular, hexagonal, oval, irregular, etc. In a furtheraspect, drive feature 54 could be a threaded female element that is ableto receive a threaded male driver member. In addition, while the drivefeature 54 is shown as a female socket, the drive feature 54 can be amale member that is capable of mating with a complementary female driverelement.

The hollow interior region 48 of the bone screw head is also configuredto receive an integrated screw locking mechanism 18. Inner surface 44 ofthe bone screw head 36 can have a variety of a mating features adaptedto receive screw locking mechanism 18 and maintain the locking mechanismtherein. In one aspect, the locking mechanism is seated within a matingfeature during assembly of the bone screw. However, the lockingmechanism could alternatively be positioned within the bone screw headby a user.

In one embodiment, the mating feature is a groove 56 that extends aroundthe circumference of inner surface 44 to receive a portion of bone screwlocking mechanism 18. One skilled in the art will appreciate that avariety of mating features, including, for example, grooves, threads,and/or raised features can be positioned with hollow interior region 48for integrating the locking mechanism in bone screw head 36. Inaddition, as discussed below, multiple mating features can be disposedon inner surface 44.

A variety of locking mechanisms can sit within the mating feature(s) ofthe bone screw head 36 and be adapted to prevent deformation and/or todeform bone screw head 36. Exemplary locking mechanisms includedisc-like locking mechanisms and locking rings. In one embodiment,locking mechanism 18 a includes a split ring that sits in a firstposition when bone screw head 36 is in an unlocked condition and sits ina second position when bone screw head 36 is a locked condition. FIG. 4Aillustrates locking mechanism 18 a in an unlocked condition in whichopen space 19 allows the diameter of locking mechanism 18 a to bereduced when a compressive force is applied. FIG. 4 shows lockingmechanism 18 a in a locked condition and space 19 closed such that thediameter of the locking mechanism cannot be further compressed.

The bone screw can have a variety of features suitable to seat a lockingmechanism such as ring 18 a. In one embodiment, inner surface 44 of bonescrew head 36 can include two or more grooves adapted to receive splitring locking mechanism 18 a. The inner surface 44, as shown in FIG. 4C,can include a first, proximal groove 56 a and a second, distal groove 56b. In one embodiment, the proximal groove 56 a has a larger diameterthen the distal groove 56 b such that when the locking mechanism isseated in groove 56 a, the locking mechanism is in an unlocked condition(FIG. 4D), and when the locking mechanism is seated in groove 56 b thelocking mechanism is in a locked condition (FIG. 4E). Other usefulfeatures suitable to seat locking mechanism 18 a include helical groovesand threads.

When the locking mechanism is in an unlocked condition, space 19 allowslocking mechanism 18 a to be compressed and tabs 52 to deflect inward toreduce the diameter of bone screw head 36 (FIG. 4D). However, when movedinto groove 56 b, locking mechanism 18 a conforms to the smallerdiameter of the groove and space 19 is dosed (or reduced). With space 19dosed, the inability (or diminished ability) of locking mechanism 18 ato further compress prevents tabs 52 from deforming. The bone screw headthus cannot be sufficiently deformed to allow passage through the upperdiameter 20 of aperture 16.

Movement of locking mechanism 18 a from groove 56 a to 56 b can beachieved by radially compressing locking mechanism 18 a and moving thelocking mechanism longitudinally. In an alternative embodiment, theinterior surface 44 could be threaded (not shown), and the lockingmechanism can be moved between a locked and unlocked condition byrotating the locking mechanism.

In an alternative embodiment of the locking mechanism, locking mechanism18 b is a disc-like member that includes two portions, one that isseated within the bone screw head 36 and another that performs thelocking function. FIGS. 5A and 5B illustrate an exemplary lockingmechanism that includes a distal portion 58, which mates with bone screwhead 36, and a proximal portion 60, which includes locking features suchas protrusions 62. The distal portion 58 can be seated within a singlegroove 56 (FIG. 3) in such a way that the locking mechanism is able torotate relative to the bone screw. In one example, the distal portion 58can be generally circular in shape.

Locking mechanism 18 b can also have a variety of alternativeconfigurations. In one aspect, locking mechanism 18 b could be invertedsuch that the proximal portion 60 of locking mechanism 18 b mates withthe bone screw head, and the distal portion 58 contains the lockingfeatures. In yet another aspect, the locking mechanism can be in theform of a member with a single portion that both mates with the bonescrew head and includes locking features.

The relative size of proximal and distal portions 60, 58 can be adaptedto permanently seat locking mechanism 18 b within groove 56. Forexample, as shown in FIG. 56, a maximum width W of proximal portion 60can be slightly less than the diameter D of distal portion 58 to providedistal portion 58 with an offset O. The offset O defines the portion ofdistal portion 58 that extends into groove 56 and holds lockingmechanism 18 b within hollow interior 48. In one exemplary embodiment,the offset is in the range about 0.1 mm to 0.5 mm.

The maximum width of proximal portion 60 of locking mechanism 18 b canbe sized to contact wall 42 when the locking mechanism is rotated intothe locked position. For example, proximal portion 60 can have agenerally circular shape with protrusions 62 that form cam-like lobes.Protrusions 62 can be sized such that when they are in the lockedposition they abut inner surface 44 of wall 42 to either prevent theouter wall from radially deforming under a compressive force or toradially expand the outer wall. In an exemplary embodiment placing thelocking mechanism in the locked position prevents the head fromdeforming (i.e., from deflecting inwardly). However, one skilled in theart will appreciate that the bone screw and locking mechanism can beconfigured such that the locking mechanism operates by causing anincrease in the diameter of the head when it is placed in the lockedcondition, as discussed below.

The size of protrusions 62, is defined as the different between W, themaximum width of proximal portion 50 and the minor width of proximalportion (FIG. 5B). The size of protrusions 62 corresponds to the maximumamount of deflection which deflectable tabs 52 can achieve. In oneembodiment, the protrusions 62 have a size in the range of approximately0.2 mm to approximately 0.7 mm.

FIG. 6A illustrates a bone screw head with locking mechanism 18 b in thelocked position. As shown, protrusions 62 are positioned in contact withdeflectable tabs 52. In this position, the tabs 52 are prevented fromdeflecting in response to a compressive force due to the positioning ofprotrusions 62. The unlocked position can be achieved by rotating thelocking mechanism such that the protrusions 62 are aligned with theslots 50 formed between deflectable tabs 52. In this position, shown inFIG. 6B, deflectable tabs 52 are free to deflect inwardly in response toa compressive force. As discussed above, in an exemplary embodiment,inward deflection of the tabs reduces the outer diameter of bone screwhead 36 and allows the bone screw head to pass into aperture 16 throughthe top portion 22 thereof, which has a reduced diameter.

Locking mechanism 18 b is dimensioned and positioned within the bonescrew 12 such that when a compressive force is applied to bone screwhead 36, the distal portion 58 of locking mechanism 18 b does notinterfere with the deformation of tabs 52. For example, diameter D ofdistal portion 58 can be such that that locking mechanism 18 b isallowed some play within the groove 56. The diameter of the groove canbe larger than diameter D of the locking mechanism disposed within thegroove. The difference in the diameters allows some space between theouter edge of distal portion 58 and the inner surface of groove 56. Whenbone screw head 36 begins to deform, this space allows outer wall 42 tocompress without immediately encountering, and being prevented fromdeforming by, the distal portion 58 of locking mechanism 18 b.

As noted above, in another embodiment, locking mechanism 18 a or 18 bcan be adapted to expand bone screw head 36 to lock bone screw head 36within aperture 16. For example, locking mechanism 18 b can bedimensioned such that the maximum width W of proximal portion 60 islarger than the internal diameter of the hollow interior 48. Rotatingprotrusions 62 into position behind tabs 52 would thus expand bone screwhead 36 to occupy the larger central diameter 30 of aperture 16 andthereby prevent bone screw head 36 from passing through the top portion22 of the aperture which has upper diameter 20. As discussed above, themagnitude of screw head deformation depends on the materials from whichthe bone screw is made and the size and/or intended use of the bonescrew. In one aspect, the bone screw head can expand by approximately0.1 to approximately 0.5 mm to lock the bone screw head in aperture 16.

Locking mechanism 18 a or 18 b can also include other features such as adrive feature adapted to mate with a complementary portion of a drivertool for shifting the locking mechanism between a locked and an unlockedposition. For example, locking mechanism 18 b can include a drivefeature 66 adapted to receive a driver tool for rotating the lockingmechanism between a locked and an unlocked position. As shown in FIGS.5A through 6B, drive feature 66, like drive feature 54 in bone screwhead 36 (FIG. 3), can be in the form of a female opening adapted toreceive a drive element on a driver tool. For example, drive feature 66can be a rectangular (e.g., square) opening that is centrally formed inthe locking mechanism. One skilled in the art will appreciate that avariety of drive features can be used to rotate locking mechanism 18 b.For example, drive feature 66 can be a female opening having a varietyof other shapes (e.g., triangular, rectangular, hexagonal oval,irregular, etc) capable of mating with the driver element. Further, asdiscussed with respect to the drive feature 54 in bone screw head 36,the drive feature 66 can alternatively be configured as a male memberthat mates with a complementary female drive element on a driver tool.

In one embodiment, both the locking mechanism 18 b and the bone screwhead 36 can be adapted to receive a single driver tool for installingbone screw 12 within bone and rotating locking mechanism 18 b betweenthe locked and unlocked positions. For example, the drive feature 66 canbe positioned coaxially with and proximal to drive feature 54, and itcan be sized such that distal drive feature 54 can be accessed throughthe proximally positioned drive feature 66. A single driver tool canthen access both drive features 54, 66 and perform the steps ofimplanting the bone screw and locking the locking mechanism withoutremoving the driver tool. As such, a surgeon can implant the bone platesystem with fewer steps while using fewer tools.

One such exemplary driver tool 70 is illustrated in FIGS. 7A through 7C.As shown driver tool 70 can include an elongate body having proximalfirst and second handle portions 74, 76 and corresponding distal firstand second driver elements 78, 80 adapted to mate with drive feature 54and drive feature 66, respectively. The driver elements 78, 80 areshaped and sized to mate with drive features 54, 66 of bone screw 12 andlocking mechanism 18 b, respectively. For example, the first matingportion 78 is shaped for insertion into the bone screw 12 (FIG. 3),while the larger, proximal mating portion 80 is adapted to mate withdrive feature 66 of the locking mechanism.

In one embodiment, an outer body sheath 82 connects handle portion 76and driver element 80. The outer body sheath 82 is positioned over shaft83 which connects handle 74 to driver element 78. The bone screws can beinstalled by mating the driver element 78 with the drive feature 56within the screw head. Rotation of the handle 74 will in turn cause thebone screw to rotate so that it can be driven into bone. Since neitherthe shaft 83 nor handle 74 is mechanically linked to sheath 82, rotationof handle 74 will not cause sheath 82 or driver element 80 to rotate.Once bone screw 12 is implanted, and bone screw head 36 is seated withinaperture 16, a surgeon can then rotate only handle 76 on shaft 82causing the driver element 80 to rotate independent of driver element78, and lock the locking mechanism within the bone screw head.

FIGS. 7B and 7C illustrate the independent movement of the first andsecond mating portions that facilitate actuation of the locking member.For example, FIG. 7B shows the second driver element 80 in an unlockedposition, while FIG. 7C shows the second driver element 80 rotated 45degrees relative to the first driver element 78 to rotate lockingmechanism 18 b into a locked position.

To assist with locking the locking mechanism, a visual indicator or astop can signify when the locking mechanism is positioned in the lockedposition. For example, a marker on the locking mechanism could bepositioned to line up with a corresponding marker on the bone screw headwhen the locking mechanism is rotated into the locked position. In use,a surgeon would line up the markers to lock the bone screw in theaperture. A pair of markers could alternatively be positioned on drivertool 70 to indicate the relative position of driver element 78, 80 andthus the locked or unlocked condition of bone screw 12. In anotherembodiment, a stop could be placed inside the bone screw head to preventrotation of the locking mechanism past the locked position. The stop,for example, could allow rotation of the locking mechanism from anunlocked position to an adjacent locked position, but not allow thelocking mechanism to rotate further. In another embodiment, the stop canbe located in the driver tool to limit rotation of the outer sheathrelative to the inner shaft. Rotation of driver element 78 relative todriver element 80 could then be limited to movement between an unlockedand an adjacent locked position.

One skilled in the art will appreciate that multiple driver tools canalso be used with bone screw 12. For example, a first driver tooladapted can be adapted to mate with drive feature 54 for implanting thebone screw, while a second driver tool can be adapted for mating withdrive feature 66 for locking the bone screw in position.

The bone plate system 10, as disclosed herein, can include a variety ofbone screw/bone plate kinematics. For example, bone plate 1 and bonescrew 1 can be adapted such that when bone screw 12 is locked in boneplate 14, the bone screw is rigidly fixed and movement of the screw inany direction is prevented. The bone plate system can also be of asemi-rigid type in which after a screw locking mechanism is engaged,screw backout is prevented, but the screw is able to move in alldirections polyaxially). Further, the bone plate system can also be of ahybrid type in which after the screw locking mechanism is engaged, screwbackout is prevented, but the screw is able to move in only one selecteddirection (e.g., the superior-inferior or the transverse direction).Moreover, the bone screws may translate within an aperture of a plate.For example, a bone screw may translate along the length of an elongatedslot defining an aperture in the plate.

The components of the exemplary bone plate systems described herein maybe constructed of any biocompatible material including, for example,metals, such as stainless steel and titanium, polymers, and compositesthereof. In certain exemplary embodiments, the bone plate system may beconstructed of a bio-resorbable material, such as, for examplepolylactic acid (PLA) and polyglycolic acid (PGA), and blends orcopolymers thereof.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims. Allpublications and references cited herein are expressly incorporatedherein by reference in their entirety.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A bone screw, comprising: an elongate memberhaving a threaded shank, and a head at a proximal end thereof, the headbeing radially deformable and being defined by an outer wall defining aninner hollow region, at least one seating groove being formed in aninner surface of the outer wall; and a screw locking mechanism at leastpartially disposed in the head, the screw locking mechanism beingrotatable within the head between a locked condition and an unlockedcondition, the screw locking mechanism being at a same axial locationwith respect to the head along a longitudinal axis of the head while inthe locked condition and in the unlocked condition; wherein the lockingmechanism has a substantially circular shaped first portion adapted tobe rotatably disposed within the seating groove, and a second portionthat includes at least one locking feature that is adapted to engage theinner surface of the outer wall when the screw locking mechanism is inthe locked condition and not engage the head when the screw lockingmechanism is in the unlocked condition, and wherein the head in thelocked configuration is radially deformed from the head in the unlockedconfiguration.
 2. The bone screw of claim 1, wherein the head has atleast one axially oriented slot formed in the outer wall and extendinginto the hollow region.
 3. The bone screw of claim 2, wherein the headhas an outer diameter that is greater than a major diameter of theshank.
 4. The bone screw of claim 2, wherein the head has four slots. 5.The bone screw of claim 4, wherein the locking mechanism has fourlocking features.
 6. The bone screw of claim 5, wherein the lockingfeatures are cam-like lobes.
 7. The bone screw of claim 6, wherein inthe locked condition the locking features engage an inner surface of theouter wall of the head to prevent deformation of the head.
 8. The bonescrew of claim 7, wherein in the unlocked condition the locking featuresalign with the slots to permit deformation of the head.
 9. The bonescrew of claim 2, wherein at least a portion of the outer wall of thehead has a spherically shaped outer surface.
 10. The bone screw of claim2, wherein the outer wall of the head has a tapered upper portion and atapered lower portion.
 11. The bone screw of claim 1, wherein theseating groove extends circumferentially about the inner surface. 12.The bone screw of claim 1, wherein the head has a drive feature formedat a distal portion of the inner hollow region.
 13. The bone screw ofclaim 12, wherein the locking mechanism has a drive feature.
 14. Thebone screw of claim 13, wherein the drive feature in the lockingmechanism and the drive feature in the head are coaxial.
 15. The bonescrew of claim 13, wherein the drive feature in the locking mechanism isa rectangular female opening having an area larger than the area of thedrive feature of the head.
 16. The bone screw of claim 12, wherein thedrive feature in the head is a rectangular female opening.
 17. The bonescrew of claim 1, wherein threads are disposed on an inner surface ofthe outer wall.
 18. A bone screw, comprising: an elongate member havinga threaded shank, and a head at a proximal end thereof, the head beingradially deformable and being defined by an outer wall defining an innerhollow region, and the head having a groove formed in the inner hollowregion thereof; and a screw locking mechanism that is rotatable withinthe head between a locked condition and an unlocked condition, the screwlocking mechanism including a distal disc-shaped lip seated within thegroove when the screw locking mechanism is in the locked condition andwhen the screw locking mechanism is in the unlocked condition, such thatthe screw locking mechanism is at a same axial location with respect tothe head along a longitudinal axis of the head while in the lockedcondition and in the unlocked condition, and the screw locking mechanismincluding a proximal portion including a protrusion proximal to the lipthat is configured to engage the head when the screw locking mechanismis in the locked condition and not engage the head when the screwlocking mechanism is in the unlocked condition; and wherein the maximumwidth of the proximal portion is larger than the internal diameter ofthe inner hollow region when the screw locking mechanism is in theunlocked condition.
 19. A bone screw, comprising: an elongate memberhaving a threaded shank, and a head at a proximal end thereof, the headbeing radially deformable, the head being defined by an outer walldefining an inner hollow region, and the head having a channel formed inthe inner hollow region thereof; and a screw locking mechanism havingproximal and distal portions, a maximum width of the proximal portionbeing smaller than a maximum width of the distal portion, the distalportion being at least partially disposed in the channel formed in thehead and being rotatable therein, the head being in a firstconfiguration when the proximal portion of the screw locking mechanismis in a first position relative to the head, the head being in a secondconfiguration when the proximal portion of the screw locking mechanismis in a second position relative to the head, the second position beingrotated from the first position about a longitudinal axis of the shank,and the head in the first configuration being radially deformed from thehead in the second configuration; wherein the screw locking mechanism isat a same axial location with respect to the head along a longitudinalaxis of the head when the head is in first configuration and when thehead is in the second configuration.